1. Field of the Invention
The present invention relates to techniques for screening integrated circuit dies during the manufacturing process. More specifically, but without limitation thereto, the present invention relates to statistical methods for screening integrated circuit dies by collecting data for parametric variables to determine a pass/fail point.
2. Description of the Prior Art
In the manufacture of integrated circuits, a large number of dies are typically formed on a semiconductor wafer. Many of these dies have defects that occur during the fabrication process, and it is desirable to detect and remove defective die from a lot before the added expense of packaging the dies into complete integrated circuits.
Normal process variations generally produce an intrinsic probability distribution for a measured parameter, for example, Vdd(min) and IDDQ. A defect may be detected by its effect on the measured parameter. For example, a short between transistors may result in an increase in quiescent current (IDDQ) that differs significantly from a mean value of quiescent current for the non-defective dies on the wafer. Other defects may be detected by a change in the minimum supply voltage Vdd(min) required by a die to meet performance specifications. The supply voltage below which a die fails to meet performance specifications is called the pass/fail point.
Previous approaches to determine the pass/fail point for an integrated circuit die typically use a standard binary search. In a standard binary search, the search range is divided into a number of equal intervals depending on the desired resolution. For example, if Vdd(min) is to be measured over the range from 1.20 V to 1.80 V with a resolution of 0.01 V, then the number of test intervals N is given by (1.8−1.2)/(0.01)=60. The number of binary search steps I is given by rounding the value of log2 N to the next highest integer. In this example, log2 60=5+, therefore I=6. A maximum of six steps are therefore needed to arrive at any interval within the test range from either end of the test range.